The present invention relates to the class of angular rate sensors wherein two waves propagate or travel in opposite directions in a closed-loop path and which may include a biasing system for frequency separating the propagating waves to minimize the effects of lock-in caused by energy coupling between the waves. In particular, the present invention provides an apparatus for providing a signal indicative of incremental lock-in error and a correction scheme whereby the sensor output can be corrected or compensated for accumulated error.
In a simple laser angular rate sensor, sometimes referred to as a ring laser gyro, two counter-traveling waves are provided by two waves or beams of substantially monochromatic electromagnetic radiation, usually two monochromatic beams of light usually in the form of laser beams. Two light beams are generated so as to travel in opposite directions along an optical closed-loop path which typically, though not necessarily, encloses the input axis about which rotation is to be sensed. When the sensor is at rest, the optical closed-loop usually referred to as the lasing path is identical for the oppositely traveling beams resulting in the frequency of oscillation of each beam being identical. Rotation of the ring laser gyro, particularly rotation of the closed-loop path, about the input axis causes the effective lasing path length traveled by one beam to increase, while the effective lasing path length traveled by the other beam to decrease. The resulting change in the effective lasing path length of the two beams produces a frequency change in each of the beams, one increasing and the other decreasing, since the frequency of oscillation of the beam of electromagnetic radiation in such systems is dependent upon the effective length of the lasing path. The frequency difference between the two beams is therefore indicative of rotation rate of the closed-loop path. The frequency difference between the two beam results in a phase difference between the counter-traveling beams which changes at a rate proportional to the frequency difference. Thus, the accumulated phase difference between the two beams is proportional to the time integral of the rotation of the closed-loop path. The total phase difference over a time interval is, therefore, indicative of the total angular displacement of the closed-loop path during the integrated time interval, and the rate of change of phase difference thereof is indicative of the rate of rotation of the closed-loop path.
A bothersome characteristic of the ring laser gyro is "lock-in". At rotation rates of the closed-loop path below some critical value called the lock-in threshold or lock-in rate, the frequency of the oppositely traveling beams synchronize to a common value resulting in the frequency difference being zero indicating no rotation at all. The lock-in characteristic arises due to mutual coupling of energy between the oppositely traveling waves. The dominant source of the coupling is mutual scattering of energy from each of the beams into the direction of the other. The effect is similar to lock-in coupling effects which have been long understood in conventional electronic oscillators.
In order to operate the gyro and provide useful rotation information, laser gyros known in the art have been provided a varying bias so as to maintain rotation of the gyro at rates above the lock-in level. A major advancement in this area was disclosed in U.S. Pat. No. 3,373,650, wherein a biasing system was provided which introduced a varying bias in the frequency of at least one of the counter-traveling beams of electromagnetic energy causing a varying frequency between the oppositely traveling beams, the bias being such that the varying frequency difference alternates in sign. The frequency bias so provided is such that their exists a frequency difference between the two oppositely traveling beams which is greater than the frequency difference which occurs near the lock-in rate for a majority of the time. Sign or polarity of the frequency difference is alternated, typically periodically, so that the time integrated frequency difference between the two beams integrated over the time interval between sign reversals reversing from the same sign direction is substantially zero. Note that at those instances of time that the sign or direction of the frequency difference reverses, the two beams will tend to lock-in since at some point the frequency difference therebetween is zero. Since the gyro output angle is generally derived from the frequency difference, an error accumulates in the gyro output angle. The periods of time that the two beams are locked-in usually are very short time intervals, and any possibly resulting gyro output angle error resulting therefrom is greatly reduced. Nevertheless, the incremental lock-in error resulting from these periods of time during lock-in corresponding to each sign reversal of the frequency difference accumulates in the gyro output angle signal, and in time can amount to a bothersome level, particularly in navigational systems. The accumulation of incremental lock-in error sometimes referred to as a random walk or random drift.
The bias provided in such biasing systems as disclosed in U.S. Pat. No. 3,373,650, is sometimes referred to as dither, and a ring laser gyro having such dither is referred to as a dithered gyro which is well known in the art. Hereafter, a dithered gyro is one in which a frequency bias is introduced into at least one of the counter-traveling beams whereby the frequency difference between the beams varies with time and alternates in sign. The alternation in sign may not be periodic in nature, i.e. not perfectly repetitious. The bias introduced may be provided by inertial rotation of the gyro (mechanical dithering) or may be provided by directly affecting the counter-traveling beams (electrical or optical dithering).
Many improvements have been made to the basic dither gyro disclosed in U.S. Pat. No. 3,373,650. One such improvement is disclosed in U.S. Pat. No. 3,467,472 wherein the improvement consists of randomly changing the amount of bias introduced into the counter-traveling beams in order to reduce the random walk resulting from the lock-in phenomenon. Accordingly, hereafter, a dither gyro is one with simple dithering or that which has dithering plus random biasing.